What is the most common location for a skin puncture?

The information given here supplements that given in Chapter 2. Users of these guidelines should read Chapter 2 before reading the information given below. This chapter covers background information (Section 7.1), practical guidance (Section 7.2) and illustrations (Section 7.3) relevant to capillary sampling.

Capillary sampling from a finger, heel or (rarely) an ear lobe may be performed on patients of any age, for specific tests that require small quantities of blood. However, because the procedure is commonly used in paediatric patients, Sections 7.1.1 and 7.1.2 focus particularly on paediatric capillary sampling.

The finger is usually the preferred site for capillary testing in an adult patient. The sides of the heel are only used in paediatric and neonatal patients. Ear lobes are sometimes used in mass screening or research studies.

Selection of a site for capillary sampling in a paediatric patient is usually based on the age and weight of the patient. If the child is walking, the child's feet may have calluses that hinder adequate blood flow. Table 7.1 shows the conditions influencing the choice of heel or finger-prick.

Specimens requiring a skin puncture are best obtained after ensuring that a baby is warm, as discussed in Section 6.2.2.

A lancet slightly shorter than the estimated depth needed should be used because the pressure compresses the skin; thus, the puncture depth will be slightly deeper than the lancet length. In one study of 52 subjects, pain increased with penetration depth, and thicker lancets were slightly more painful than thin ones (67). However, blood volumes increased with the lancet penetration and depth.

Lengths vary by manufacturer (from 0.85 mm for neonates up to 2.2 mm). In a finger-prick, the depth should not go beyond 2.4 mm, so a 2.2 mm lancet is the longest length typically used.

In heel-pricks, the depth should not go beyond 2.4 mm. For premature neonates, a 0.85 mm lancet is available.

The distance for a 7 pound (3 kg) baby from outer skin surface to bone is:

medial and lateral heel – 3.32 mm;
posterior heel – 2.33 mm (this site should be avoided, to reduce the risk of hitting bone);
toe – 2.19 mm.

The recommended depth for a finger-prick is:

for a child over 6 months and below 8 years – 1.5 mm;
for a child over 8 years – 2.4 mm.

Too much compression should be avoided, because this may cause a deeper puncture than is needed to get good flow.

With skin punctures, the haematology specimen is collected first, followed by the chemistry and blood bank specimens. This order of drawing is essential to minimize the effects of platelet clumping. The order used for skin punctures is the reverse of that used for venepuncture collection. If more than two specimens are needed, venepuncture may provide more accurate laboratory results.

Complications that can arise in capillary sampling include:

collapse of veins if the tibial artery is lacerated from puncturing the medial aspect of the heel;
osteomyelitis of the heel bone (calcaneus) (68);
nerve damage if the fingers of neonates are punctured (69);
haematoma and loss of access to the venous branch used;
scarring;
localized or generalized necrosis (a long-term effect);
skin breakdown from repeated use of adhesive strips (particularly in very young or very elderly patients) – this can be avoided if sufficient pressure is applied and the puncture site is observed after the procedure.

7.2.1. Selection of site and lancet

Using the guidance given in Section 7.1, decide whether to use a finger or heel-prick, and decide on an appropriate size of lancet.
DO NOT use a surgical blade to perform a skin puncture.
DO NOT puncture the skin more than once with the same lancet, or use a single puncture site more than once, because this can lead to bacterial contamination and infection.

Prepare the skin

Apply alcohol to the entry site and allow to air dry (see Section 2.2.3).
Puncture the skin with one quick, continuous and deliberate stroke, to achieve a good flow of blood and to prevent the need to repeat the puncture.
Wipe away the first drop of blood because it may be contaminated with tissue fluid or debris (sloughing skin).
Avoid squeezing the finger or heel too tightly because this dilutes the specimen with tissue fluid (plasma) and increases the probability of haemolysis (60).
When the blood collection procedure is complete, apply firm pressure to the site to stop the bleeding.

Take laboratory samples in the correct order to minimize erroneous test results

With skin punctures, collect the specimens in the order below, starting with haematology specimens:
–
haematology specimens;
–
chemistry specimens;
–
blood bank specimens.

Immobilize the child

First immobilize the child by asking the parent to:
–
sit on the phlebotomy chair with the child on the parent's lap;
–
immobilize the child's lower extremities by positioning their legs around the child's in a cross-leg pattern;
–
extend an arm across the child's chest, and secure the child's free arm by firmly tucking it under their own;
–
grasp the child's elbow (i.e. the skin puncture arm), and hold it securely;
–
use his or her other arm to firmly grasp the child's wrist, holding it palm down.

Prepare the skin

Prepare the skin as described above for adult patients.
DO NOT use povidone iodine for a capillary skin puncture in paediatric and neonatal patients; instead, use alcohol, as stated in the instructions for adults.

Puncture the skin

Puncture the skin as described above for adult patients.
If necessary, take the following steps to improve the ease of obtaining blood by finger-prick in paediatric and neonatal patients:
–
ask the parent to rhythmically tighten and release the child's wrist, to ensure that there is sufficient flow of blood;
–
keep the child warm by removing as few clothes as possible, swaddling an infant in a blanket, and having a mother or caregiver hold an infant, leaving only the extremity of the site of capillary sampling exposed.
Avoid excessive massaging or squeezing of fingers because this will cause haemolysis and impede blood flow (60).

Take laboratory samples in the order that prevent cross-contamination of sample tube additives

As described above for adult patients, collect the capillary haematology specimen first, followed by the chemistry and blood bank specimens.
Collect all equipment used in the procedure, being careful to remove all items from the patient's bed or cot; to avoid accidents, DO NOT leave anything behind.

There are two separate steps to patient follow-up care – data entry (i.e. completion of requisitions), and provision of comfort and reassurance.

Data entry or completion of requisitions

Record relevant information about the blood collection on the requisition and specimen label; such information may include:
–
date of collection;
–
patient name;
–
patient identity number;
–
unit location (nursery or hospital room number);
–
test or tests requested;
–
amount of blood collected (number of tubes);
–
method of collection (venepuncture or skin puncture);
–
phlebotomist's initials.

Show the child that you care either verbally or physically. A simple gesture is all it takes to leave the child on a positive note; for example, give verbal praise, a handshake, a fun sticker or a simple pat on the back.

A small amount of sucrose (0.012–0.12 g) is safe and effective as an analgesic for newborns undergoing venepuncture or capillary heel-pricks (70).

Adhere strictly to a limit on the number of times a paediatric patient may be stuck. If no satisfactory sample has been collected after two attempts, seek a second opinion to decide whether to make a further attempt, or cancel the tests.

Capillary blood sampling is becoming a common way to minimize the amount of blood drawn from a patient. The 10–300 microliters can be used to look for anemia, check blood sugar or even to evaluate thyroid function. The procedure is easier and less painful than traditional venipuncture which draws blood from a vein.

In order to collect blood with a capillary tube, the appropriate site must be cleaned and punctured with a lancet so that a drop of blood can be gently expressed.

But what is an appropriate site?

Finger — Usually the third or fourth finger is preferred in adults and children. The thumb has a pulse and is likely to bleed excessively. The index finger can be calloused or sensitive and the little finger does not have enough tissue to prevent hitting the bone with the lancet. The puncture is done to the left or right of the midline of the palmar surface of the fingertip, staying away from the fingernail.
Heel — This site is used for infants, less than one year old, whose fingers are too tiny. The puncture is done on the farthest lateral or medial aspect of the plantar surface of the heel, not on the bottom. Punctures done on the plantar surface can damage cartilage or bone.
Ear lobe — The ear lobe has been used for capillary blood sampling, but is no longer recommended. Studies have shown that the blood flow in the ear lobe is less than that in either the finger tip or the heel. The ear lobe can still be used if no other site is available.
Great toe — The big toe can be used if necessary in infants who are not yet walking. Callous formation after a child starts walking can interfere with blood collection and is a contraindication. The site should be lateral to the midline of the plantar surface of the toe. Only the great toe has the amount of tissue necessary to protect the bone from injury.
Palm — There are two areas on the palm that can be used if necessary. Especially in diabetics who test their own blood sugars frequently, alternate sites give some relief from reusing the same sites over and over. The thenar and hypothenar eminences have capillary circulation equivalent to the fingertip.

Other sites such as the abdomen or thigh have been used, but are not generally recommended.